Digitizing CCD array system

ABSTRACT

A digitizing CCD array system calibrates the CCD array precisely, masks a data medium automatically, and recovers data from the data medium accurately. The system uses a substantially opaque film mask to eliminate the effects of diffused light and flare during a digitization process when the data medium is smaller than an illumination source. A multiple optical density filters are used in calibrating the CCD array to provide a precise optical density reference for digitization of the data medium. In addition, the invention increases contrast sensitivity in data recovery by selectively switching the CCD array on and off to control the duration of multiple exposure times, i.e., integration periods, during a single line scan.

This application is a divisional application of U.S. patent applicationSer. No. 08/569,453, filed Dec. 8, 1995 now U.S. Pat. No. 5,682,033.

FIELD OF THE INVENTION

This invention relates to a charge coupled device (CCD) array fordigitizing data on a data medium, and in particular, a system forcalibrating the CCD array, masking the data medium from extraneousillumination, and converting data on the data medium from analog todigital representation.

BACKGROUND OF THE INVENTION

Like most optical sensors, a CCD array responds linearly to lightintensity. A charge on the CCD array is directly proportional to thenumber of light photons striking a particular sensor. However, due tothe electronics, the optics, the light source, and the sensor itself, aCCD array with many sensors (5000 for example) does not responduniformly during data recovery and conversion to electric charge levels.This non-uniformity requires each sensor in the array to be calibratedor normalized to every other sensor through electronic and softwarecompensation. This calibration is accomplished by determining the nearsaturation level (light output) and the dark level (quiescent output) ofeach sensor. These two points provide means to mathematically normalizethe linear output of all sensors in the array by interpolating betweenthe two values. This is based on an assumption that a linearrelationship exists between an electric charge level and lightintensity.

Data on a data medium having an optical density (OD) range of 0.0 to 6.0is typically recovered using sensors which have a dynamic opticaldensity of 3.0 decibels, employing multiple light levels and exposuretimes. In this case, there has been no automatic means to accuratelycalibrate the sensors to reference the output digital values to opticaldensities of the data medium.

Unfortunately, the CCD array sensors do not always exhibit the linearrelationship beyond optical density of 3.0 decibels. In fact, majordeviations occur near the dark level output. This non-linearity causesdeviation in calibration, and consequently makes any reference tooptical density inaccurate which may interfere with the data recoveryfrom the data medium, i.e., film.

Furthermore, the CCD array for reading data from the data mediumnormally possesses a fixed field of view. When reading data from thedata medium which is smaller than the fixed field of view of the CCDarray, the extraneous illumination beyond the edges of the data mediumyields undesirable flare. The flare increases the charge of all sensorsin the array reducing the reliability of the process. To reduce theunwanted effect of this flare, manual masking is usually employed aroundthe edges of the data medium.

As stated above, the CCD sensor possesses limited dynamic range ofoptical density due to the limited number of photons that can beaccumulated in each sensor. The number of photons is directlyproportional to the intensity of illumination reaching each sensor.Since

    I=I.sub.0 /10.sup.OD, where

I is the intensity of transmitted illumination,

I₀ is the intensity of incident illumination, and

OD is optical density of the data medium,

the intensity of transmitted illumination is reduced by a factor of 10for each integral value of optical density, provided that the intensityof the transmitted illumination remains constant.

For example, 99% of the light is absorbed by a data medium of opticaldensity 2.0. Thus, the ability to discriminate one optical density fromanother, i.e., contrast sensitivity, is extremely reduced after only 2decades of optical density, leaving only 1% of the light for logarithmicdistribution among the remaining optical densities.

To recover data from a data medium possessing greater than 2 decades ofdensity with significant contrast sensitivity, several succeeding scansof the data medium at different light levels and/or differentintegration periods are necessary. Multiple succeeding scans aretime-consuming and cumbersome due to the retrieval and re-insertion ofthe data medium.

A need, therefore, exists for a digitizing CCD array system whichovercomes the above disadvantages of the existing digitizing systems.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to calibrate each sensor in aCCD array accurately to obtain a precise relationship of an electriccharge level versus light intensity.

It is another object of the invention to recover data from an opticallyread data medium by eliminating the effects of diffused light and flareduring a digitization process.

It is yet another object of the invention to improve contrastsensitivity when reading data from an optically read data medium duringa digitization process.

SUMMARY OF THE INVENTION

These and other objects, features and advantages are accomplished by thepresent invention.

In one aspect of the invention, a digitizing CCD array system forrecovering data includes an illumination source which generates a lightbeam to illuminate the data medium, providing an image of the datamedium on the CCD array which produces an electric signal having amagnitude proportional to the incident light intensity. Adjacent thedata medium is a substantially opaque film, hereinafter referred to as a"mask", which is located in the plane of the data medium. The maskpartially shields the illumination source from illuminating areasoutside the data medium to prevent flare during the digitizationprocess.

Further in accordance with another aspect of the invention, a motordisplaces the mask via a pair of sprockets on a shaft. The sprocketstranslate the mask to partially block the light beam. A lens is locatedin a path of the light beam which illuminates the data medium and themask. The lens focuses the light beam onto the CCD array which convertsthe focused light beam into an electric charge representing informationrecorded on the data medium.

In another aspect of the invention, a light beam from the illuminationsource traverses the data medium and then passes through the lens. Thelens focuses the light beam onto the CCD array which includesintegration control means for alternately switching the charge coupleddevice on and off during a single line scan of the data medium. Thealternate switching of the CCD produces a multiple of exposure timeperiods, resulting in the CCD array accumulating a different chargelevel for each exposure time period. Connected to the CCD array areprocessing means which, for each pixel, process multiple digitalrepresentations corresponding to each of the multiple exposure timeperiods. A single digital representation is then selected from thesemultiple digital representations, which corresponds to an opticaldensity of the data medium. Thus, the multiple images are merged into asingle image by an algorithm that provides a harmonious blending whichyields a wide dynamic range with excellent contrast sensitivity.

In yet another aspect of the invention, calibration of the invention isimplemented by an optical filter mechanism comprising filters of variousoptical densities. The optical filter mechanism is positioned betweenthe film and the mask for calibrating the CCD array for each of themultiple exposure time periods.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned as well as additional advantages and features of thepresent invention will be evident and more clearly understood whenconsidered in conjunction with the accompanying drawings, in which:

FIG. 1 is an illustration of one embodiment of the disclosed digitizingCCD array system.

FIG. 2 is a block diagram of electronic components of the discloseddigitizing CCD array system, which process electric charge levelsaccumulated in the CCD array.

FIG. 3 is a more detailed block diagram of the electronic components ofthe disclosed digitizing CCD array system.

FIG. 4 is a graph depicting the difference between an ideal and actualCCD sensor responses.

FIG. 5 is a graph showing the response (optical density versus digitalvalue) of a single line scanned twice using different integrationperiods.

FIG. 6 is a graph showing the response (optical density versus digitalvalue) of a single line scan merged from the dual integration scans ofFIG. 5.

In all Figures, like reference numerals represent the same or identicalcomponents of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

First, an overview of components in one embodiment of the discloseddigitizing CCD array system is presented during a general operation ofthe system with reference to FIGS. 1 and 2.

FIG. 1 is an illustration of one embodiment of the disclosed digitizingCCD array system. A light source, such as a line illuminator 10,generates a light beam across the width of a data medium 12 from whichthe information is recovered. The data medium 12 is typically scanned ona line-by-line basis by moving the data medium 12 in the direction ofthe X--X axis 11, as indicated in FIG. 1.

In one embodiment of the invention, a mask mechanism 68 is positionedadjacent the data medium 12. The mask mechanism 68 comprises a motor 18which includes a shaft 20 and a pair of sprockets 16 and 16' fixedlyattached to the shaft 20. The shaft 20 and the sprockets 16 and 16' arerotated by the motor 18 during its operation. The sprockets 16 and 16'engage a substantially opaque film mask 14 for transporting it partiallyacross the width of the data medium 12, as shown by the dash lines 9 ofFIG. 1. Under the operation of the motor 18, the mask 14 can also beretracted to a non-functional position of not covering any portion ofthe data medium 12 or the line illuminator 10. As clearly seen in FIG.1, the mask 14 can be fully extended to cover the width of the datamedium 12 and the line illuminator 10. The second mask mechanism 68',partially shown in FIG. 1, exists on the opposite side of the lineilluminator 10.

An optical density filter mechanism 69 is interposed between a lens 22and the data medium 12. This mechanism 69 includes a filter mechanismmotor 61 which rotates a filter wheel 60. The filter wheel 60 holdsmultiple filters, among them a near zero optical density filter 62, atypical mid-range filter 63, and another filter 64. In this exemplaryembodiment, the filters 63 and 64 are selected to have an opticaldensity of 2.0 and 4.0, respectively.

The light beam from the line illuminator 10 passes through the datamedium 12, as well as the mask 14 in certain situations, and is focusedby the lens 22. The focused light beam then strikes a CCD array 24 whichaccumulates the photons. The accumulated photons are then converted bythe CCD array 24 into electric charges representing recorded informationof the data medium 12. The electric charges accumulated in the CCD array24 represent a line of information simultaneously scanned across thedata medium 12. To scan the next line of information recorded on thedata medium 12, typically the data medium 12 is shifted in the directionof the X--X axis 11, and the process is repeated from the beginning.

FIG. 2 is a block diagram of the electronic components which process thephotons and electric charge levels accumulated in the CCD array 24.

In general, the CCD array 24 typically accumulates photons to itscapacity as long as the light beam illuminates the array'sphotosensitive surface. When the accumulated photons reach thepredetermined capacity level of the individual sensors, a signal 27 istransmitted to the CCD array 24 to stop accumulating the photons. Afterthe individual sensors reach the saturation level, no furtherinformation can be obtained from the CCD array 24 even if the sensorscontinue to be exposed to the illumination. While the signal 27 ismaintained, the CCD array 24 is precluded from receiving additionalphotons even though exposed to the light beam. The accumulation ofphotons by the CCD array 24 is effectively prevented as long as thesignal 27 is active. In the disclosed system, a timer 28 and a clock 26automatically control the CCD array 24, as will be explained later inthe description.

Continuing with the description of FIG. 2, the signal 27 stops the CCDarray 24 from accumulating photons striking the photosensitive surface.Regardless of whether the CCD array 24 reached the saturation level, theaccumulated photons are converted into electric charges. The electriccharges are then output to an analog-to-digital (A/D) converter 30 via aline 31 for conversion to a digital representation. From the A/Dconverter 30, the digital representations of various charge levelscorresponding to the information on the data medium 12 are sent toprocessing electronics 32 via a line 33 for further processing of theinformation.

Next, the detailed operation of the disclosed system is described withreference to FIGS. 1 and 2. Prior to recovering data from the datamedium 12, the digitizing system must be calibrated to accuratelyrepresent the recorded data due to a difference between the ideal andactual responses as shown in FIG. 4. Using the apparatus and methoddescribed herein, the disclosed system achieves the accurate calibrationto provide an optical density reference for the data medium 12. Bysubtracting a near quiescent response value of each sensor andmultiplying by a scale factor, each sensor can be normalized to respondsimilarly to every other sensor.

Referring to FIG. 1, to calibrate the digitizing CCD array system, thedata medium 12 is initially removed from the light beam of the lineilluminator 10. In addition, the motor 18 is activated to retract themask 14 to its original, non-functional position of not covering anypart of the line illuminator 10. Next, the near zero optical densityfilter 62 is introduced into a path of the light beam. The filter 62 isattached to the filter wheel 60 and positioned by the filter mechanismmotor 61.

The line illuminator 10 is then turned on to illuminate the CCD array 24with a light beam emitted from the line illuminator 10. In response tothe light beam emitted from the line illuminator 10 and focused by thelens 22, the CCD array 24 acquires, via the conversion of the photons, afirst numerical quantity of electric charges.

During the accumulation of charges, the intensity of the light beam isheld constant, while duration of the exposure time, i.e., integrationperiod, is controlled by the timer 28 which activates the signal 27 tothe CCD array. The integration period, selected as 0.3 msec in oneembodiment of the present invention, is empirically found such that anysensor in the CCD array 24 reaches the near saturation value. In the16-bit digital representation, the saturation value is 65536. Theconversion of photons to electric charges and from analog to digitalrepresentation produces the scale factor for each sensor. Thus, thescale factors for various sensors in the CCD array 24 may be65536/65500, 65536/65510, etc. The processor 32 subsequently stores thescale factor for each sensor responsive to the first illumination. Afterthe transfer of the electric charges, the sensors become available foraccumulation of another electric charge level.

The filter mechanism motor 61 is subsequently activated to bring thefilter 63 of optical density 2.0 into the path of the light beam. Whilethe line illuminator 10 is maintained at the previous intensity, thetimer 28 is again activated for 0.3 msec integration period. The signal27 is de-asserted enabling the CCD array 24 to start accumulatingphotons.

At the expiration of the 0.3 msec integration period controlled by thetimer 28, the signal 27 is activated to prohibit further accumulation ofphotons. The electric charge level is then converted to a digital valueby the A/D converter 30. This digital value becomes the offset factorfor each sensor in the CCD array 24.

Using the previously stored scale factor at the optical density of 0.0and the offset factor at the optical density of 2.0, the look-up tableis created for the 0.0-2.0 optical density range. Each sensor can thusbe normalized for the optical density range 0.0-2.0. For example, whilerecovering the actual data from the data medium 12, a sensor may producea digital value corresponding to 31,200 in the decimal notation. Thisvalue is then adjusted by subtracting the offset factor and multiplyingby the scale factor, respectively.

Next, after creating the look-up table for the 0.0-2.0 optical densityrange, an analogous calibration procedure is employed for normalizingsensors in the 2.0-4.0 optical density range. The filter 63 of 2.0optical density is maintained in its position while the line illuminator10 is kept at the previous intensity level. The exposure time, i.e.,integration period, is increased to a second predetermined value.Similar to the first integration period of 0.3 msec, this integrationperiod, selected as 30 msec in one embodiment of the present invention,is empirically found such that any sensor in the CCD array 24 reachesthe near saturation value.

In response to the light beam which passes through the filter 63 and isfocused by the lens 22, the CCD array 24 acquires another electriccharge level for each sensor, preceded by the conversion of the photons.This charge level is then converted to a digital form, representing ascale factor for each sensor. During this integration period, alsoterminated by the signal 27 to the CCD array 24, the intensity of thelight beam is kept substantially the same as in the previousilluminations, while the integration period is appropriately adjusted tobring the sensors in the CCD array 24 to the near saturation level.Consequently, the integration period is 100 longer in the exemplaryembodiment, because the more opaque filter 63 is now positioned betweenthe CCD array 24 and the line illuminator 10 allowing less light toreach the sensors. The sensors, therefore, must be exposed to the lightbeam for a longer time to achieve near saturation level whileaccumulating the photons.

Further in accordance with the invention, the motor 61 is activated tointroduce another filter 64 of the 4.0 optical density into the path ofthe light beam. The line illuminator 10 is again maintained at theoriginal intensity level. The timer 28, set for a 30 msec integrationperiod, deactivates the signal 27 to enable the CCD array 24 to startaccumulating photons. When the timer expires in 30 msec, the signal 27is asserted to prohibit further accumulation of photons. The A/Dconverter 30 converts the electric charge level to a digital value foreach sensor. This digital value becomes the offset factor for eachsensor in the CCD array 24 for the 2.0-4.0 optical density range.

Using the previously stored scale factor at the 2.0 optical density andthe offset factor at the 4.0 optical density, the look-up table iscreated for the 2.0-4.0 optical density range. Each sensor can thus benormalized for the optical density range 2.0-4.0.

Ideal and actual linear sensor responses are shown in FIG. 4. Bysubtracting the quiescent response (b value) of each sensor andmultiplying by a multiplication factor, each sensor can be normalized torespond similarly to every other sensor.

The above discussion described the steps for obtaining calibration ofthe sensors for the optical density ranges 0.0-2.0 and 2.0-4.0. It iswell understood, however, that if the calibration of the CCD array 24 isrequired with greater precision, additional filters of various opticaldensities between 0.0 and 4.0 can be used to normalize the sensors atadditional optical density levels. Similarly, if calibration is desiredat an optical density greater than 4.0, appropriate filters in thefilter wheel 60 may be inserted for placing into the path of the lightbeam.

Once the CCD array 24 is accurately calibrated in accordance with theprevious description, the data recovery by the disclosed digitizing CCDarray system can then begin. Again referring to FIG. 1, the data medium12 is positioned between the line illuminator 10 and the lens 22. Theline illuminator 10 generates the light beam which passes through thedata medium 12.

In some instances, however, the width of the data medium 12 as indicatedby the dash line 13, in the direction of the Y--Y axis 15, may benarrower than the line illuminator 10. Such condition results inilluminating extraneous areas outside the scan line of the data mediumby the light beam which is emitted from the line illuminator 10. Theflare from illuminating the outside areas adversely affects thesensitivity of other sensors in the CCD array 24 as described above. Toshield the areas outside the data medium 12 from illumination, thesubstantially opaque mask 14 is positioned in the plane of the datamedium 12. The mask 14 is transported by the motor 18 driving the shaft20 and the sprockets 16 and 16' which engage the mask 14. The mask 14 ispartially positioned over the line illuminator 10 to selectively coverits portions. Although still illuminating the areas outside the datamedium 12, the light beam passing through those areas is effectivelyblocked by the opacity of the mask 14 from reaching the CCD array 24. Byusing the motor 18 to extend the mask 14 across the line illuminator 10,the mask 14 can be easily adjusted to match any width of the data medium12 if it is narrower than the line illuminator 10.

Regardless of the need for the masking operation, the data recovery fromthe data medium 12 proceeds as follows. After passing through the datamedium 12, the light beam is focused by the lens 22. The focused lightbeam strikes the CCD array 24 which accumulates the photons from thefocused light beam. The CCD array 24 gathers the photons until it eithersaturates or receives the signal 27 to stop accumulating the photons.

In the disclosed invention, the CCD array 24 accumulates the photonsfrom the light beam under the control of the clock 26 and the timer 28,as shown in FIG. 2. The clock 26 can be a crystal or other precise pulsegenerating means which continuously generates clock pulses 29. The timer28 receives the clock pulses 29 and increments its internal count onevery pulse. Upon reaching a predetermined count which can be selectedvia programming means, the timer 28 generates the signal 27 to stop theCCD array 24 from accumulating any additional photons from theilluminating light beam.

Integration period of the sensors is effectively controlled during asingle line scan. To obtain a better contrast sensitivity, the disclosedsystem generates several integration periods of various duration duringa single line scan. These integration periods correspond to the exposuretimes previously used during the calibration process of the sensors.

For example, during the first exposure of a single line of the datamedium 12, the timer 28 counts for 0.3 milliseconds and produces thesignal 27 upon reaching this terminal count. At this time, the sensorsof the CCD array 24 stop accumulating photons. This integration periodcorresponds to the previously calibrated optical range 0.0-2.0. Thesecond exposure of 30 milliseconds for the same line corresponds to thepreviously calibrated optical range 2.0-4.0.

FIG. 3 shows a more detailed representation of electronic components inthe disclosed invention. The photons are stored in a capacitive well ineach individual sensor, such as sensors S1, S2, S3, etc. of the CCDarray 24. The photons in each sensor are converted by the CCD array 24into electrical charges in each sensor. The processor 32 then transfersthe electric charge level in each sensor to the A/D converter 30.

Further continuing with FIG. 3, under the control of the processor 32,the digital values from the A/D converters are stored in a plurality ofregisters R1, R2, etc. of a memory buffer 36. Each register R1, R2, etc.stores the digital value of each sensor represented by a predeterminednumber of bits of information, such as 8, 12, 16, depending on thedesired resolution. The digital values from the memory buffer 36 arethen transferred alternately line-by-line to two identical memory banks,such as 40 and 42 located in a host system 50, so that the two imagesare produced: one representing data starting from 0.0 OD and anotherstarting from 2.0 OD. Each of the memory banks 40, 42 is similarlyconfigured as the memory buffer 36 for storing data from multipleintegration scans.

During the above processing, the processor 32 applies scale and offsetfactors which were previously determined during the calibration. Forexample, during the first integration time of 0.3 milliseconds, scaleand offset factors obtained during the calibration for 0.3 millisecondsare employed. This processing occurs for all data-bearing locations in aline of the data medium 12 which is scanned by the disclosed system. Thedigital representation, produced by each sensor and adjusted by thecorresponding scale and offset factors, is stored in the memory bank 40of the host system 50.

While the conversion and processing of the stored photons take place,the signal 27 is deactivated. When the signal 27 is inactive, thesensors resume photon accumulation. At this time, the timer 28,previously reset upon reaching the terminal count, again resumescounting the pulses 29 from the clock 26. This time, the timer 28 timesout when it reaches 30 milliseconds. The signal 27 is then generated tothe CCD array 24 stopping the sensors from accumulating additionalphotons. As described above, the stored photons in each sensor areconverted by the CCD array 24 into electric charge levels which areconverted by the A/D converter 30. The digital representation, producedby each sensor and adjusted by the corresponding scale and offsetfactors, is stored in the memory bank 42 of the host system 50.

These two exposures occur in succession during a single line scan,illuminating essentially the same line of information on the data medium12. Furthermore, the exposures correspond to the exposures during thecalibration of the CCD array 24, as stated above. As a result, the firstexposure of 0.3 milliseconds produces an electric charge level for theoptical density range 0.0-2.0 of the data medium 12. The second exposureof 30 milliseconds produces an electric charge level for the opticaldensity range 2.0-4.0 of the data medium 12. Thus, while the intensityof the light beam remains constant throughout these multiple exposures,change in the exposure duration produces illumination to the CCD array24 which favorably matches the optical density ranges of the data medium12 from which data is being recovered and converted into digitalrepresentation. The values versus optical density for the two images aredepicted in FIG. 5.

When the host system 50 receives multiple digital values during a singleline scan, it selects the largest non-saturation digital valuecorresponding to the electric charge level converted from the datamedium 12. If the host system 50, for example, receives two values of12456 and 65536, it discards the saturation value of 65536 fromconsideration. The remaining value then becomes the digitalrepresentation for the data location on the data medium 12. Undercontrol of a computer program, the host system 50 can distinguishbetween two arrays of digital values, i.e., digital values from thearray A correspond to the optical density range 0.0-2.0, digital valuesfrom the array B correspond to the optical density range 2.0-4.0, etc.Based on the association of the position of each digital value with thecorresponding optical density range, the digital value 3000 correspondsto the optical density range 2.0-4.0 and is a digital representation forthe data-bearing location on the line scanned by the disclosed system.The basic concept of the single image algorithm utilizes digital valuesin the 30 msec scan that are not saturated (less than 65536 in a 16-bitresolution, for example) and digital values in the 0.3 msec scan thatcorrespond to saturated pixels in the 30 msec scan. This is graphicallyshown in FIG. 6, where two images of FIG. 5 are merged into a singleimage providing a harmonious blending of the two sets of values.

The above description refers to a single location on the line of thedata medium 12. It is understood, however, that the above steps areconcurrently performed for all data-bearing locations of a single lineduring a line scan by the line illuminator 10. The data is, therefore,recovered from each data-bearing location during a single line scan,reflecting a greater contrast among various optical densities. The nextline of the data medium 12 is scanned in the same manner, until data isrecovered and converted to the digital representation for the entiredata medium 12.

Since those skilled in the art can modify the disclosed specificembodiment without departing from the spirit of the invention, it is,therefore, intended that the claims be interpreted to cover suchmodifications and equivalents.

What is claimed is:
 1. A method of recovering data from a data medium toincrease contrast sensitivity in a digitization process of a chargecoupled device array system, comprising:generating a light beam toilluminate said data medium; focusing said light beam via a lens;producing during a single line scan of said data medium a multiple ofexposures for said data medium; controlling duration of each exposure insaid multiple of exposures; converting each exposure into acorresponding electric charge level representing information recorded onsaid data medium; and processing a multiple of electric charge levelscorresponding to said multiple of exposures by selecting from saidmultiple of electric charge levels a single charge level correspondingto an optical density of said data medium.
 2. The method according toclaim 1, wherein said processing comprises determining a digitalrepresentation of information on said data medium by selecting thelargest non-saturation value from said multiple exposures.
 3. The methodof recovering data from a data medium according to claim 1 wherein saidstep of converting said exposure into a corresponding electric chargelevel comprises accumulating photons from said exposure onto an array ofcoupled devices.
 4. The method of recovering data according to claim 3wherein said step of controlling duration of said exposurecomprises:generating a time interval representing said exposure time;and inhibiting further accumulation of photons on said array of chargecoupled devices following said duration of time.
 5. The method forrecovering data according to claim 4 wherein said time interval issubstantially 0.3 milliseconds for one of said exposures.
 6. The methodfor recovering data according to claim 4 wherein said time interval issubstantially 30 milliseconds.
 7. The method for recovering dataaccording to claim 1 further comprising calibrating said array ofcharged coupled devices by:obtaining a near quiescent value for each ofsaid charge coupled devices; subtracting said quiescent value from eachelectrical charge level produced from a respective charge coupleddevice; obtaining a scale factor for each of said charge coupled devicesto normalize the value of charge produced from each of said chargecoupled devices with each other; and multiplying the charge levelproduced by each of said charge coupled devices with a respective scalefactor.
 8. The method according to claim 3 wherein said multipleexposures correspond to an optical density of said data medium of0.0-2.0 and 2.0-4.0, respectively.